In recent years, the trend toward micro-scale pattern rule has been increasing with the trend toward large-scale integration and high-speed of LSI. The trend toward a shorter wavelength of the exposure light source lies behind it. For example, it has become possible to mass-produce DRAM (dynamic random-access memory) of 64M-bit (processing dimension is 0.25 μm or less) by the wavelength shortening from mercury lamp i-line (365 nm) to KrF excimer laser (248 nm). Furthermore, in order to realize the production of DRAM having integration degrees of 1 G or greater, a lithography using ArF excimer laser (193 nm) has been adopted.
As a resist suitable for such exposure wavelength, “a chemically amplified resist material” has attracted much attention. This contains a radiosensitive acid generator (hereinafter referred to as “photoacid generator”) which generates acid by radiation irradiation (hereinafter referred to as “exposure”), and serves as a pattern-forming material that forms a pattern by making a difference in solubility in a developing solution between the exposed portion and the unexposed portion through a reaction using the acid generated by exposure as a catalyst.
Also concerning a photoacid generator used for such a chemically amplified resist material, studies have variously been carried out. It has been found that an acid strength to cleave an acid labile group of resin is not sufficient in the case where a photoacid generator for generating alkane or arenesulfonic acid, as had been employed for a conventional chemically amplified resist material adopting KrF excimer laser as the light source, is used as a component of the above-mentioned ArF-type chemically amplified resist material; in which resolution cannot be done at all or the sensitivity is so poor as to be adapted to the device production.
Therefore, as the photoacid generator for the ArF-type chemically amplified resist material, those that generate perfluoroalkanesulfonic acid high in acid strength are commonly used; however, perfluorooctane sulfonic acid and derivatives thereof, which are known as PFOS abbreviated by their initials, bring about problems of stability (non-degradability) stemmed from a C—F bond, and biological concentration and accumulation stemmed from hydrophobicity or lipophilicity. Additionally, perfluoroalkanesulfonic acid having 5 or more carbon atoms and derivatives thereof also cause the above problems.
In order to address such problems regarding PFOS, partially-fluorinated alkanesulfonic acids of which degree of fluorine substitution is reduced have been under development at all locations. For instance, onium alkoxycarbonylfluoromethanesulfonates such as triphenylsulfonium methoxycarbonyldifluoromethanesulfonate (Patent Publication 1), (4-methylphenyl)diphenylsulfonyl t-butoxycarbonyldifluoromethanesulfonate (Patent Publication 2) or triphenylsulfonium (adamant-1-ylmethyl)oxycarbonyldifluoromethanesulfonate (Patent Publication 3) has been developed as an acid generator.
On the other hand, triphenylsulfonium 1,1,3,3,3-pentafluoro-2-benzoyloxypropane-1-sulfonate, which is a kind of an onium alkylcarbonyloxyalkanesulfonate and has an ester bond opposite to that of the above-mentioned onium alkoxycarbonyldifluoromethanesulfonate, and the like have been developed (Patent Publication 4).
The present applicant has found an onium 2-alkylcarbonyloxy-1,1-difluoroethanesulfonate having three less fluorine atoms than the acid generator of the Patent Publication 4 so as to be considered to less affect the environment, and has found that this substance functions as an acid generator exhibiting a high acid strength with the minimum possible number of fluorine atoms and has an excellent compatibility with solvents or resins so as to be useful as the acid generator for the resist material (Patent Publication 5).
In Patent Publication 5, a reaction path as represented by the following equation [1]
is disclosed as a method for synthesizing an onium 2-alkylcarbonyloxy-1,1-difluoroethanesulfonate. More specifically, the path includes: a first step of reacting bromodifluoroethanol with a carboxylic chloride thereby obtaining a corresponding ester; a second step of sulfinating the obtained ester with a sulfinating agent thereby obtaining a metal sulfinate; a third step of oxidizing the obtained metal sulfinate with an oxidizing agent thereby obtaining a metal sulfonate; and a fourth step of reacting the obtained metal sulfonate with a monovalent onium salt thereby obtaining an onium sulfonate.
Furthermore, the present applicant has found a polymerizable onium tetrafluoroalkanesulfonate, which is a similar onium alkylcarbonyloxyalkanesulfonate but nevertheless one having one less fluorine atom than the acid generator of the Patent Publication 4 so as to be considered to less affect the environment (Patent Publication 6).
In Patent Publication 6, a reaction path as represented by the following equation [2]
is disclosed as a method for synthesizing the polymerizable onium tetrafluoroalkanesulfonate. More specifically, the path includes: a first step of sulfinating 4-bromo-3,3,4,4-tetrafluorobutan-1-ol with a sulfinating agent thereby obtaining a metal sulfinate; a second step of oxidizing the obtained metal sulfinate with an oxidizing agent thereby obtaining a metal sulfonate; a third step of reacting the obtained metal sulfonate with a monovalent onium salt thereby obtaining an onium sulfonate; and a fourth step of reacting the obtained onium sulfonate with an alkyl acrylic acid halide or alkyl acrylic acid anhydride thereby obtaining a target polymerizable onium sulfonate.
Additionally, a similar onium tetrafluoroalkanesulfonate is disclosed also in other publication (Patent Publication 7). In this publication, 1,4-dibromo-1,1,2,2-tetrafluorobutane is prepared as the starting material and is converted into a 4-bromo-3,3,4,4-tetrafluorobutyl ester of aliphatic or aromatic carboxylic acid by undergoing selective substitution reaction using a carboxylic acid salt such as sodium carboxylate and ammonium carboxylate. The ester is then reacted with a sulfinic acid-forming agent such as sodium dithionite in a solvent such as water, acetonitrile and a mixture of these in the presence of a base such as sodium hydrogencarbonate thereby obtaining a 4-acyloxy-1,1,2,2-tetrafluorobutanesulfinic acid salt, just as Patent Publication 6. Thereafter, it was customarily oxidized with an oxidizing agent such as aqueous hydrogen peroxide in water serving as a solvent in the presence of sodium tungstate or the like.